The invention relates to thermal cut-off switches for electrically operated devices, and particularly to such switches that are resettable.
Many electrically operated tools and appliances should not be operated above certain temperatures because of the harm to users that can result or because of potential damage to the tool or appliance itself. The problem is particularly important for electrical heating appliances such as toasters and hair-treating appliances, where a minor malfunction of the unit's circuitry can lead to the dangerously excessive generation of heat. Accordingly such electrical appliances usually incorporate thermally triggered cut-off switches in the electric circuits supplying power to the devices. The switches cut off power to the device when a sensor reaches a pre-set temperature.
It is desirable to make such a switch as inexpensively as possible. The reliability of the switch is, nevertheless, the prime consideration. Since producing a thermal cut-off switch that is resettable usually requires additional elements and more careful manufacture, one solution to meeting the requirement for both economy and reliability is to produce a switch that operates just once. Such a solution is presumably based on the assumption that most devices do not overheat, and that if they do and the switch is "used up" in cutting off power, the device will require repair and replacement of the switch can be part of the repair. Making the switch reusable is desirable if it must be reset often, or if it can be produced as economically and reliably as a non-resettable switch.
A common kind of thermal cut-off switch includes connecting elements that are biased apart but are held together by an arrangement of elements dependent on the solidification of metable material. When a hazardous temperature is reached the meltable material liquifies and the connecting elements are dis-engaged. See, e.g., U.S. Pat. No. 3,820,050. In single use switches, such as the one shown in that patent, the meltable material is dissipated when it melts and no reset function is provided. In resettable switches, it is necessary to devise an arrangement that preserves the meltable material and provides for a re-establishment of the original switch arrangement. U.S. Pat. No. 3,444,355 shows an arrangement in which ratchets holding a disengagement plunger are kept from motion by solid fusible material securing the ratchets to their housing. The material seems to be contained in the housing so that if the material melts, allowing the ratchets to turn and release the plunger, when it later resolidifies, it secures the ratchets again and allows the plunger to be reset and held. An economical and efficient switch should have a less complicated arrangement, fewer parts, and a surer way of containing reusable meltable material so that it can be used again and again, than shown in these examples of the prior art.
Accordingly, it is an object of the invention to provide a thermally operated electrical cut-off switch in which a mass of meltable material plays the active role, that is economical to make, reliable in action, and resettable.
Other objects of the invention will in part be obvious and will in part appear hereinafter.